The present invention relates to flowmeters generally, and more particularly to measuring chambers for oscillating piston type flowmeters.
A conventional measuring chamber for a flowmeter is shown in FIGS. 1 and 2 in which flow of a fluid through the measuring chamber causes ring piston 2 to oscillate, while the center of the piston is constrained by piston pin 4 and inner cylindrical wall 6 to revolve through a circular orbit concentric with outer cylindrical wall 8 of the chamber. Inner and outer walls 6, 8 define an annular chamber 10 in which the piston revolves and oscillates. Partition wall 12 together with ring piston 2 divides the annular chamber into receiving and discharging spaces. The receiving space is filled by fluid entering inlet port 14, and the fluid is discharged through outlet port 16 at each complete oscillation. In order to permit the piston to move in the desired oscillating path, the side wall 18 of the piston includes slot 20 so that side wall 18 fits over partition 12 against which the side wall bears and forms a seal during oscillation. The piston also includes web 22 which is provided with radial slot 24 in a configuration that permits the necessary pivoting movements of the piston. Opening 25 is the uppermost opening in the row of openings formed through the side wall of the piston along one side of slot 20. Another similar row of openings is formed on the other side of slot 20, but hidden from view. These openings permit some fluid flow between the interior of the piston and the annular chamber such that when the piston moves away from the partition wall, low pressure areas are avoided. The outer cylindrical wall also is provided with a vertical row of openings (not shown) adjacent inlet port 14 for similar reasons. Sealing pin 26 extends from cylindrical wall 6 to form a seal between the piston and partition wall 12.
A take-off mechanism (not shown) for registering the volume of fluid passed through the metering chamber per oscillation of the piston ring is coupled to piston pin 4. Piston pin 4 is centrally positioned in piston web 22 and includes upper portion 4a which is coupled to the take-off mechanism and lower portion 4b which extends below web 22 into annular channel 27 and engages control roller or bushing 28. Control roller 28 is loosely carried by pin or stud 30 which is fixed to bottom head 32 of the measuring chamber. It is known to use brass to make the control roller and the lower portion of the piston pin which contacts the control roller, and to make the fixed stud of hardened steel. When the softer collar wears, it is simply replaced. However, among the drawbacks of these flowmeters is that when the piston revolves at a relatively high velocity, pin 4 tends to move radially outward due to centrifugal forces. This causes the inner and outer side surfaces of piston wall 18 to contact the inner and outer cylindrical walls 6, 8 of the measuring chambers at diametrically opposed positions on the piston as designated by reference numerals 34 and 36, such that undesirable wear of the piston and inner and outer cylindrical walls results. The wear increases the clearance between the piston and inner and outer walls 6, 8, which causes the capillary seal between the piston and inner and outer walls 6, 8 to deteriorate, thereby reducing meter accuracy. It also has been found that at relatively high piston velocities, significant deflection or cupping of piston web 22, piston nutation and vibration occur. It is believed that the cupping and nutation results from the tendency for generally unconstrained portion 4b of pin 4 to deflect perpendicular to the center axis of the pin. In addition to causing the piston to contact and wear the inner and outer cylindrical walls, piston cupping, vibration and nutation cause the piston to displace the capillary seal between the piston and inner and outer cylindrical wall which causes fluid leakage which, in turn, reduces meter accuracy.